Process and arrangement for converting an acoustic signal to an electrical signal

ABSTRACT

To facilitate direct conversion to digital form of an acoustic signal acting on the acoustic receptor of an acoustic receiver while satisfying requirements of dynamic range, noise and adequate quantization, the following is proposed: the acoustic receptor should be exposed to a counter-signal when the acoustic signal acts on it in such a way that the acoustic receptor is largely maintained in its rest state despite the action of the acoustic signal. The counter-signal is derived from the control variable of a control circuit which is a component of the acoustic receptor. The control variable contains the information on the acting acoustic signal. Any deviation of the receptor from its rest state immediately generates a digital “nought” or “one.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a sound receiving process and to a soundreceiving arrangement.

2. Description of Related Art

Efforts made so far to design a “true” digital microphone without analogintermediate step have not proceeded past theoretical ideas. On thebasis of these ideas, the position or movement of a sound receptor (e.g.a diaphragm) for an electroacoustic sound source is measured, eitheroptically or by means of ultrasound, e.g. by evaluating interferencepatterns or transit time effects, wherein a counting operation is used,among other things, to digitize the measured information. Such a processif disclosed, for example, in the GB-A-1 077 074. The sound is picked upvia two sound receptors, which are connected in series acoustically inthe direction of the incoming sound. The signal voltages given off bothsound receptors are displaced by an amount that follows from the soundtransit time between the two sound receptors, which are arranged at aspecified distance. By comparing and digitizing these two signals, a1-bit DPCM signal is generated, which is then transmitted to an up/downcounter for conversion to a bit-parallel digital signal.

Converters have in the meantime become available for the purelyelectrical conversion from analog audio signals to a correspondingdigital signal, which for the most part meet the special requirementsfor converting audio signals. Above all, this refers to a highresolution, linearity and low inherent noise. In particular, Sigma-Deltaconverters achieve these characteristics, as is disclosed, for example,in the references U.S. Pat. No. 5,181,032 and U.S. Pat. No. 5,191,332.With the known Sigma-Delta converters, the audio signal is fed into acontrol circuit, wherein the feedback counter-coupling signal isconducted via a 1-bit or a traditional multi-bit AD converter and acorresponding inverse converter. In the generated digital 1-bit ormulti-bit data current, the analog audio signal information isrepresented by the time ratio of the digital 0/1 states. The desireddigital output signal is obtained by means of digital filtering andreformatting. Such a control circuit system represents a modulator thatis synchronized with a supplied clock pulse, wherein favorable noise andresolution qualities are achieved by splitting the information in themodulator into several signal paths and a varied signal treatment.

However, all known converters for generating a digital signal from anacoustic signal are unsuitable for studio microphones because theycannot compete with analog studio microphones with respect to dynamicrange, noise level and sufficient quantization.

SUMMARY OF THE INVENTION

In contrast, it is the object of the invention to specify a process anda sound receiving arrangement, which makes it possible to directlyconvert an acoustic signal, acting upon the sound receptor of a soundreceiver, to a digital information, thereby meeting the requirementswith respect to dynamic range, noise and sufficient quantization.

This object and other objects are addressed by the inventive process andapparatus.

In a first embodiment, the invention comprises a process for convertingan acoustic signal, acting upon the sound receptor of a sound receiver,to an electrical signal. In the inventive method, if the acoustic signalacts upon the sound receptor, the sound receptor is also acted upon by acounter-signal, such that the sound receptor remains mostly in itsresting state. The counter-signal is derived from a control variable ofa control circuit of which the sound receiver is a component. Thecontrol variable contains information on the acoustic signal, and eachdeviation of the receptor from its resting state generates digitalinformation (“0” or “1”).

In a further embodiment, the invention comprises a sound receivingarrangement. The sound receiving arrangement comprises a sound receiverincluding a sound receptor. In the inventive sound receivingarrangement, if the acoustic signal acts upon the sound receptor, thesound receptor is also acted upon by a counter-signal, such that thesound receptor remains mostly in its resting state. The counter-signalis derived from a control variable of a control circuit of which thesound receiver is a component. The control variable contains informationon the acoustic signal, and each deviation of the receptor from itsresting state generates digital information (“0” or “1”).

The invention is based on the idea of retaining the capacitive converterprinciple, unsurpassed so far with respect to dynamic scope and noisebehavior, of a “true” digital microphone. The known and maturetechnology of the capacitive converter can be fully incorporated withthis. The capacitive converter is incorporated into a digitizingconversion process, in such a way that the receptor (e.g. a capacitordiaphragm), upon which the acoustic signal acts as sound pressure, isnot deflected proportional to the signal strength, but according to theinvention is kept almost in the rest state through a counter-actingsound signal or a counter force. The counter-signal is derived from thecontrol variable of a control circuit, of which the sound receiver is acomponent, wherein the control variable contains the information on theacoustic signal. As compared to the known capacitor microphones andowing to the fact that the receptor for the most part remains in itsreverberative rest state, characteristic errors that depend on thereceptor position and lead to signal distortions, as well as mechanicalself-resonances of the receptor that influence the frequency course andthe impulse behavior of the electrical output signal for all practicalpurposes are no longer effective. Also, the invention practically nolonger requires measures for a passive damping of the receptor, such asare required for linearizing known capacitor microphones by taking intoaccount a reduction in the sensitivity, so that the sensitivity of aconverter designed according to the invention is clearly improved. It isessential that the remaining slight deflections of the receptor areevaluated so as to provide only information on the direction of thedeviation from the rest state and that this information is displayed as“zero” or “one.” It means that the comparator function as elementaryfunction of each analog/digital conversion process is carried outdirectly at the sound receptor, without requiring an analog intermediatesignal obtained from the sound receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail with the aid of theexemplary embodiments shown in the drawings, wherein:

FIG. 1 shows a block diagram of a first embodiment of a digitalmicrophone according to the invention;

FIG. 2 shows a block diagram of a second embodiment of a digitalmicrophone according to the invention;

FIG. 3 shows a block diagram of a third embodiment of a digitalmicrophone according to the invention; and

FIG. 4 shows a block diagram of a first embodiment of a digitalmicrophone according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIGS. 1 to 4, the reference 1 denotes a sound source and thereference 2 a sound receiver, which can be at the same location or atdifferent locations and can be based on the same or differentelectroacoustic converter principles. Essential is that two opposing,but equally strong forces simultaneously act upon the sound receptor ofsound receiver 2, namely the force of the incoming active sound(acoustic signal) and the counter force of a counter-signal, generatedby sound source 1, which results in the effect intended according to theinvention of keeping the sound receptor for the most part in the reststate, despite the effect of the acoustic signal. Even the smallestdeviation of the receptor from its rest state in positive or negativedirection can be evaluated immediately as digital information “one” or“zero.” Thus, the digital information is formed directly at the receptorfor sound receiver 2.

The counter-signal is derived from the control variable of asufficiently fast control circuit containing sound source 1 and soundreceiver 2 as components, so that the sound source 1 can generate acounter-signal, which arrives simultaneously with the acoustic signal atthe sound receiver and has the same value as the acoustic signal. Theacoustic transit time or the structural distance between sound source 1and sound receiver 2 here are crucial for determining the achievablefrequency band width of the control circuit and should therefore be assmall as possible, so as to ensure a stable control circuit operationfor the complete audible frequency range. Consequently, it makes sensein practical operations to have sound source 1 and sound receiver 2 inthe same location, which is equivalent to having the sound receptor(e.g. the diaphragm) of sound receiver 2 and the sounder of sound source1 combined to form a single component, that is to say sound source 1 andsound receiver 2, for example, have a joint diaphragm. It is furthermoreadvantageous if sound source 1 and sound receiver 2 operate on the basisof different electroacoustic converter principles to avoid anundesirable electrical bypass and thus a cross-talk interference. Forexample, the sound source 1 can be realized electrostatically ormagnetically and the sound receiver 2 as the capacitor of ahigh-frequency resonant circuit.

The exemplary embodiments shown in FIGS. 1 to 3, differ in the manner inwhich the digital information, generated directly at the receptor forsound receiver 2, is evaluated as well as in the design of the controlcircuit.

In the embodiment according to FIG. 1, the control circuit is designedin the form of a modified Delta-Sigma modulator, e.g. as described inthe magazine “Audio Professional,” issue 3/4, 1995, pages 59 to 65.

The sound receiver 2 is realized in FIG. 1, as well as in all the otherFIGS. 2 to 4, as capacitor of a high-frequency resonant circuit withresonant circuit inductivity 22. Owing to the incoming active sound, thejoint diaphragm of the sound source/sound receiver combination ½ isinitially deflected and detunes the HF resonant circuit through thechanging capacity. The resonant circuit inductivity 22 is a component ofa high-frequency demodulator 3 (phase demodulator or amplitudedemodulator), which is indicated by a HF oscillator 37 and a demodulatordiode 36 in the HF demodulator 3 unit. A long modulation characteristic,as is needed for traditional capacitor microphones, is not necessary forthe HF demodulator 3 since it is only necessary to detect the correctmathematical signs relating to the deviations of the diaphragm for thesound source/sound receiver combination ½ in positive or negativedirection, starting from the rest state. The HF demodulator 3 thereforecan be designed to have a very high sensitivity, which is a considerableadvantage with respect to the noise and dynamic behavior of the totalsystem.

The output signal for HF demodulator 3 is supplied to a comparator 4,the output signal of which electrically represents the digitalinformation that is generated directly at the receptor (diaphragm) forsound receiver 2, that is to say it reproduces the deviation in thediaphragm position in positive or negative direction as “0” signal or“1” signal. This digital signal represents a 1-bit word. In order togenerate from this a multi-bit word, a 4-bit word for the example shownhere, the output signal of comparator 4 controls the counting direction(up/down input) of a 4-stage counter 5. The clock input CLK of thiscounter is clocked by a clock generator 9 (CTL network), which isclocked, for example, with 64 times the scanning frequency (FS) of 48kHz that is standard for the digitizing of audio signals. As a result ofthe excess scanning with 64 times 48 kHz (=3,072 MHZ), the timeresolution of the 1-bit word, represented by the ratio of “zeros” to“ones,” is increased corresponding to the degree of excess scanning. A4-bit signal develops at the parallel outputs A, B, C and D of counter5, which signal contains the information on the amplitude for theincoming acoustic signal at sound receiver 2. However, the quantizationof the information is not only amplitude oriented (4-bit word). Owing tothe excess scanning of the 1-bit word at the counter 5 input, thequantization of the information is also time-oriented, corresponding tothe temporal relationship between various 4-bit words.

The 4-bit word at the parallel outputs of counter 5 is on the one handsupplied to a digital filter 10 and, on the other hand, to a 4-bitdigital/analog converter 6. The 4-bit signal that has been converted toan analog signal is routed through a single-stage or multi-stageup-integration and difference formation by means of a chain ofdifferencing and integrating stages 7.1 to 7.N, in order tostatistically distribute the bit patterns, developed during thequantization process, in the frequency transmission range and toconcentrate the quantization noise in a frequency range above theaudible frequency range. The signal developing at the end of the chainof differencing and integrating stages 7.1 to 7.N is amplified in adriver amplifier 8, the output signal of which drives the sound source1. The control circuit composed of components 2, 3, 4, 5, 6, 7.1 to 7.N,8, and 1 is herewith closed. As previously mentioned, the forces actingupon the diaphragm as a result of the incoming sound are neutralizedowing to the effect of this control signal.

The digital filter 10 with its parallel inputs A, B, C and D wherein the4-bit word coming from the parallel outputs of counter 5 is present, isclocked with the same clocking frequency (3,072 MHz) as the counter 5.The filter 10 serializes the parallel 4-bit word, wherein a 20-bitsignal 12 with a scanning frequency of 48 kHz appears at the output ofdigital filter 10 as a result of the 64-times excess scanning. A FIRfilter preferably is provided as digital filter 10. Furthermore, thenoise portions in the 4-bit output signal of counter 5, which are abovethe audible range, are effectively suppressed during the digitalfiltering.

It is understood that the serial digital 20-bit output signal 12 canalso be converted to other optional data formats. With respect to this,FIG. 1 indicates a format converter 11 with a serial input SER.IN towhich the signal 12 is supplied. The clocking input CLK and anadditional input FRM CTL for the synchronizing of words, are connectedto the clock generator 9. The optionally provided format converter 11generates a parallel output signal at its multiple outputs, of which thefirst one is given the designation LSB (corresponding to leastsignificant bit) and the last one the designation MSB (corresponding tomost significant bit). The format converter 11 furthermore has an outputAES/EBU for an AES/EBU interface, as well as a free output OTHER FORMfor selecting a different digital format.

In a modification of the embodiment according to FIG. 1, the controlcircuit can be designed as a 1-bit converter, so that by omitting thecounter 5, the comparator 4 output is connected directly to the chain ofdifferencing and integrating stages 7.1 to 7.N. Furthermore, it is notnecessary to first demodulate the modulated HF oscillation and thendigitize it (by means of HF demodulator 3 with series-connectedcomparator 4). Rather, it can be converted directly to a (digital) 1-bitsignal in a stage 30, as shown in FIGS. 2 and 3. The stage 30 contains alimiter amplifier or comparator 31, which converts the phase-modulatedHF oscillation at the resonant circuit coil 22 directly to a square-wavesignal with digital logic level. A further component is the phase-lockedHF clock oscillator 33, which stimulates the resonant circuit,consisting of the capacitive sound receiver 2 and the resonant circuitcoil 22, via the coupling capacitor 35, and which is synchronized by theclock oscillator 9, if necessary. The 1-bit signal sequence is generateddirectly through a digital phase comparison between the digitized HFoscillation and the HF clock oscillator 33, which signal sequencecarries the information of the sound receptor deflection from the reststate. In the embodiment under review, in FIGS. 2 and 3, this functionis executed by a D-FlipFlop. The 1-bit signal is subsequently reqad intothe digital filter 10 with the necessary excess scanning, from which thedesired quantization of the active signal results, and is then suppliedto the differencing and integrating stages 7.1 to 7.N.

The embodiment according to FIG. 3 differs from the embodiment accordingto FIG. 2 in that the differentiating and integrating stages 7.1 to 7.Nwith digital filter 10, which are typical for a Delta-Sigma converter,are omitted and are replaced by a high-resolution digital/analogconverter 60, so that the control circuit is closed once more. In thatcase, the digital output signal 12 develops directly at the output fordigital/analog converter 50, which signal is shown as a serial signal inthe example viewed, and which can be converted in the format converter11 to optionally formatted, digital output signals in the previouslydescribed manner.

FIG. 4 shows an improved analog microphone, so to speak as a “byproduct”of the digital microphone according to FIGS. 1 to 3, for which only thesound receiver/sound source combination ½, the HF demodulator 3, and thedriver amplifier 8 were retained as compared to the circuit arrangementaccording to FIG. 1. The demodulated HF signal (with a very smallamplitude) at the output of HF demodulator 3 is amplified simply bymeans of an amplifier 20 in order to form an analog, high-qualitymicrophone output signal 23. Furthermore, the driver signal for drivingthe sound source 1 is generated in amplifier 9 from the output signal23. If desired, the analog output microphone-output signal 23 can beconverted to a digital signal by means of a traditional analog/digitalconverter 21, which digital signal is shown as a serial signal in theembodiment shown here. Of the advantages of the “true” digitalmicrophone according to FIGS. 1 to 3, the digital microphone accordingto FIG. 4, which is reconfigured as an analog microphone, retains theadvantages of an insignificant sound receptor deflection and therewithconnected, above-explained improvements with respect to linear andnon-linear distortions, as well as the sensitivity, provided theamplifier 20 is designed to provide a sufficiently high amplification.With an amplification factor of 100 for amplifier 20, for example, thediaphragm deflection of the sound receiver 2, as well as the electricaloutput signal of sound receiver 2 are reduced by a correspondingmeasure.

What is claimed is:
 1. A method of converting an acoustic signal to anelectrical signal, the acoustic signal acting upon a sound receptor of asound receiver, the method comprising the steps of: generating at leastone digital information signal when the acoustic signal acts upon thesound receptor; processing said at least one digital information signalin a control circuit to obtain a control variable, wherein the controlcircuit includes the sound receiver; generating a counter-signal basedon the control variable; and applying the counter-signal to the soundreceptor, wherein the counter-signal acts to neutralize forces exertedupon the sound receptor by the acoustic signal, whereby the soundreceptor is made to remain primarily in an equilibrium state.
 2. Themethod according to claim 1, wherein the step of generating acounter-signal comprises the step of coupling the control variable to asound source, wherein the sound source is acoustically coupled to thesound receiver.
 3. The method according to claim 1, wherein thecounter-signal is generated by a sound source, and wherein the soundreceptor is provided to function simultaneously as a sound-receiving andsound-emitting component of a sound source/sound receiver combination.4. The method according to claim 1, wherein the sound receptor comprisesa diaphragm.
 5. The method according to claim 1, wherein the soundreceptor comprises at least one of a resiliently positioned componentand a component designed as a spring.
 6. The method according to claim1, wherein the step of generating a counter-signal comprises the step ofcoupling the control variable to a sound source, and wherein the soundsource and the sound receiver are designed according to electro-acousticconverter principles.
 7. The method according to claim 1, wherein thestep of generating at least one digital information signal comprises thestep of: converting sound receptor deflections to a digital signal usinga comparator.
 8. The method according to claim 7, wherein the step ofgenerating a counter-signal comprises the step of: converting thedigital signal to an analog signal using a digital/analog converter. 9.The method according to claim 7, wherein the step of processingcomprises the step of: filtering the digital information signal in sucha way that time information is transformed to amplitude information. 10.The method according to claim 9, wherein the step of filtering thedigital information signal comprises the step of using a digital filterto filter the digital information signal.
 11. The method according toclaim 9, further comprising the step of: converting the filtered digitalinformation signal to a different data format.
 12. The method accordingto claim 11, wherein the step of filtering the digital informationsignal comprises the step of using a digital filter to filter thedigital information signal.
 13. The method according to claim 1, whereinthe control circuit operates based on the principle of a Delta-Sigmamodulator, and wherein the receptor is included in a comparator functionof the Delta-Sigma modulator.
 14. The method according to claim 13,wherein the Delta-Sigma modulator is a one-bit Delta-Sigma modulator.15. The method according to claim 13, wherein the Delta-Sigma modulatoris a multi-bit Delta-Sigma modulator.
 16. The method according to claim1, further comprising the step of: modulating at least one of the phaseand amplitude of an HF signal generated by a resonant circuit, acapacitative component of which is the sound receiver, when the acousticsignal acts upon the sound receptor.
 17. The method according to claim16, further comprising the step of: demodulating the HF signal using anHF demodulator.
 18. The method according to claim 16, wherein the HFsignal is phase modulated, and further comprising the steps of: directlydigitizing the phase-modulated HF signal using a limiter comparator; andconverting the directly digitized phase-modulated HF signal directly toa digital signal carrying information from the sound receptor using aphase comparator.
 19. A method of converting an acoustic signal to anelectrical signal, the acoustic signal acting upon a sound receptor of asound receiver, the method comprising the steps of: generating an analogelectrical signal in response to the acoustic signal acting upon thesound receptor; amplifying the analog electrical signal to produce anamplified analog signal; supplying the amplified analog signal to asound source to generate a counter-signal; and applying thecounter-signal to the sound receptor, wherein the counter-signal acts toneutralize forces exerted upon the sound receptor by the acousticsignal, whereby the sound receptor is made to remain primarily in anequilibrium state.
 20. The method according to claim 19, furthercomprising the step of: converting the amplified analog signal to adigital signal.
 21. A sound receiving apparatus comprising: a soundreceiver having a sound receptor; means for generating an electricalsignal when an acoustic signal acts upon the sound receptor; a controlcircuit for processing the electrical signal, wherein the controlcircuit includes the sound receiver, the control circuit generating acontrol variable; means for generating a counter-signal based on thecontrol variable; and means for applying the counter-signal to the soundreceptor, the counter-signal acting to maintain the sound receptorprimarily in an equilibrium state by neutralizing forces exerted uponthe sound receptor by the acoustic signal.
 22. The apparatus accordingto claim 21, wherein the means for applying the counter-signal to thesound receptor comprises a sound source.
 23. The apparatus according toclaim 22, wherein the sound receptor functions simultaneously as asound-receiving component and as a sound-emitting component of a soundsource/sound receiver combination.
 24. The apparatus according to claim21, wherein the sound receptor comprises a diaphragm.
 25. The apparatusaccording to claim 21, wherein the sound receptor comprises at least oneof a resiliently positioned component or a component configured as aspring.
 26. The apparatus according to claim 21, further comprising: acomparator for converting sound receptor deflections to a digitalsignal.
 27. The apparatus according to claim 26, further comprising: adigital-to-analog converter for converting the digital signal into ananalog signal.
 28. The apparatus according to claim 21, the controlcircuit operating on the principle of a Delta-Sigma modulator, whereinthe sound receptor is included in a comparator function of theDelta-Sigma modulator.
 29. The apparatus according to claim 21, whereinthe means for generating an electrical signal comprises: an HF resonantcircuit generating an HF signal, the HF resonant circuit including acapacitative component, wherein the sound receptor modulates the HFsignal.
 30. The apparatus according to claim 29, wherein the HF resonantcircuit includes said sound receiver as the capacitative component. 31.The apparatus according to claim 29, further comprising: an HFdemodulator, coupled to the HF resonant circuit, for demodulating themodulated HF signal.
 32. The apparatus according to claim 29, whereinthe type of modulation is phase modulation, and further comprising: alimiter comparator for digitizing the phase-modulated HF signal toproduce a digitized HF signal; and a digital phase converter forconverting the digitized HF signal to a digital signal containinginformation received from the sound receptor.
 33. The apparatusaccording to claim 21, wherein the control circuit includes a digitalfilter processing a digital signal to produced a filtered digitalsignal, so as to convert time information into amplitude information.34. The apparatus according to claim 33, wherein the digital filtercomprises an FIR filter.
 35. The apparatus according to claim 33,wherein the control circuit further includes: means for converting thefiltered digital signal to a different data format.
 36. The apparatusaccording to claim 35, wherein the digital filter comprises an FIRfilter.
 37. The apparatus according to claim 21, wherein the means forgenerating an electrical signal generates an analog signal, wherein thecontrol circuit comprises an amplifier for amplifying the analog signalto produce an amplified signal, and wherein the means for generating acounter-signal comprises a sound source to which the amplified signal isdirectly applied.
 38. The apparatus according to claim 37, furthercomprising: means for converting the amplified signal into a digitalsignal.